Reference is made to commonly-assigned U.S. patent application Ser. No. 10/021,410 filed Dec. 12, 2001, entitled xe2x80x9cApparatus for Permitting Transfer of Organic Material From a Donor to Form a Layer in an OLED Devicexe2x80x9d by Phillips et al, the disclosure of which is incorporated herein by reference.
The present invention relates to depositing organic material onto substrates for use in making organic light emitting diode (OLED) devices.
In color or full-color organic electroluminescent (EL) displays having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media is required to produce the RGB pixels. The basic organic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the light-emitting layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily, such as the hole-transporting layer or the electron-transporting layer. In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the light-emitting layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, there are problems of alignment between the substrate and the shadow mask, and care must be taken that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is difficult to manipulate the shadow mask to form appropriately positioned pixels. A further disadvantage of the shadow-mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of non-functioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side. It is extremely difficult to manufacture larger shadow masks with the required precision for accurately forming EL devices.
A method for patterning high-resolution organic EL displays has been disclosed in commonly-assigned U.S. Pat. No. 5,851,709 by Fleming et al. This method is comprised of the following sequences of steps: 1) providing a donor substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color-forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Fleming et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow-mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatented donor sheet and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. Such a method is disclosed by Littman in commonly-assigned U.S. Pat. No. 5,688,55 1, and in a series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553). The latter patents teach a method that can transfer, by a change in adhesion, the light-emitting layer of an EL device from a donor sheet to a substrate by heating selected portions of the donor with laser light. While this is a useful technique, there are serious difficulties in applying it on a large-scale manufacturing of EL devices. To make an EL device that includes thousandsxe2x80x94or even millionsxe2x80x94of pixels in three colors in a reasonable amount of time (a few minutes) would require a laser beam which moves very fast in two dimensions. This increases alignment problems, and also increases the possibility of causing misalignment due to vibrations from the rapidly moving machinery. A further disadvantage is that the rapid movement of the laser beam necessitates a very short dwell time on each spot to be transferred, which further necessitates a very high-powered laser.
It has been determined that the combination of short dwell time and high power can cause non-uniform spattering of the donor material and reciprocity problems, and does not allow the uniform transfer of material that would be afforded with an evaporation or sublimation process. Such a process further contributes to non-uniform transfer of donor material through uneven heating of the donor material because of the rotationally-symmetric Gaussian distribution of the laser; that is, the center of the laser has greater intensity and thus can cause the transfer of more material than the edges of the laser beam.
It is therefore an object of the present invention to provide a method of forming layers on an EL device by donor transfer in a rapid manner while eliminating problems due to short laser dwell time on the donor.
This object is achieved by a method of depositing organic layers onto a substrate in the making of an OLED device, comprising the steps of:
(a) providing a donor element having transferable organic material in transfer relationship with an OLED substrate;
(b) forming a substantially uniform linear laser light beam;
(c) providing a spatial light modulator responsive to the linear laser light beam and adapted to form multichannel linear laser light beams;
(d) individually modulating selected channels to form one or more laser light beam segments wherein each segment can include one or more laser light beam channels and further wherein the laser light beam segment(s) have substantially square intensity profiles in a first direction and a substantially Gaussian intensity profile in a second direction perpendicular to the first direction and are directed onto the donor element; and
(e) the donor element producing heat in response to the light from the modulated segments so as to heat transfer organic material onto selected areas of the substrate.
An advantage of this method is that electroluminescent panels can be produced rapidly with high quality. The overall donor transfer time is reduced while not reducing dwell time, and therefore keeping the material transfer in the evaporation or sublimation regime, and spattering is greatly reduced or eliminated in the donor material transfer. A further advantage of this method is that the substantially uniform (i.e. non-Gaussian) intensity profile of laser light beam segments in one direction maintains greater uniformity of the deposited layer across the pixel width. A further advantage is that the need for a shadow mask and all the problems inherent in its use are eliminated. A further advantage of this method is that it can maintain EL spot precision on large EL panels, which is difficult or impossible to do with existing methods. A further advantage is that the method is quickly and easily scalable to any size EL panels and/or different pixel sizes without the need to wait for a different-size shadow mask to be fabricated. A further advantage is that this method can be used to print very small emitting pixels (5 to 10 micrometers), which are difficult or impossible to make with a shadow mask technique.